Electronic assemblies and circuits comprising electronic components are typically built up on component carriers such as printed circuit boards (PCBs). A manufacturing process of a so-called multilayer component carrier typically includes filling holes in the component carrier with electrically conductive material such as copper.
Density and complexity of electronic components are constantly increasing and holes in component carriers, such as laser drilled holes, are getting smaller, while the aspect ratios of the holes are getting higher. In this respect it is a common problem that failures in the material which is used for filling these holes are produced during the component carrier manufacturing process. Failures such as cracks or voids in the filling material of the holes are often not avoidable when manufacturing component carriers. These cracks and voids lead to severe problems concerning the functionality of the electronic assembly built upon the component carrier. Consequently, the quality of signal transmission, especially in the case of high frequencies, will be significantly reduced.
The problem of crack and void formation may be inherent to the manufacturing process step of filling a hole in a component carrier as will be apparent from the following descriptions.
In general, an electro-plating process is applied in order to fill a hole in a component carrier. Therefore, an anode (positively charged) is placed into a bath in order to establish an electric field between the anode and a negatively charged entity, which should be electro-plated, and which serves as a cathode. The electric field drives cations, such as copper ions, which are dissolved in the bath, to the surface of the entity, which should be electro-plated. At the negatively charged surface, the cations will be chemically reduced and a layer of copper will start growing on the surface of the entity, which should be electro-plated. This process can be used for filling the hole. In the following, two prior art examples are described and it will be apparent from these prior art examples, how the failures within the filling material emerge.
A prior art example of a conventional process for filling a hole in a component carrier by electro-plating is illustrated in FIG. 3A and FIG. 3B. FIG. 3A shows a hole 320 in a component carrier 300, wherein the hole 320 is physically delimited by walls 330a and 330b, and a bottom structure 333. Hereby, the bottom structure 333 and the surfaces of the walls 330a and 330b should be electro-plated to thereby fill the hole 320 with copper. It can be clearly seen in FIG. 3A that copper material 310 starts growing from the left wall 330a, the right wall 330b, and from the bottom structure 333. However, it can also be clearly seen that the growing of the copper material 310 takes place in an irregular manner. In the center of each copper material portion 310, at the walls 330a and 330b, and at the bottom structure 333, the growing speed is faster, and consequently the thickness of the copper material 310 is higher, than for the copper material 310 at the outsides of the walls 330a and 330b, and the bottom structure 333. Growing directions are hereby indicated by arrows. As a result, gaps 345 remain at the transition regions between the walls 330a and 330b, and the bottom structure 333 of the hole 320.
When the growing process is finished and the hole is filled with copper 310′, failures 350 such as cracks and/or voids will remain in the center of the filled hole 320′, as can be seen in FIG. 3B. These failures 350 directly result from the irregular growing process of the copper material 310′, because the gaps 345 remain during the growing process and finally result in cracks and/or voids, which cannot be repaired anymore.
Conventionally, such failures and the accompanied problems are accepted as unavoidable. Some manufacturers apply chemical additives in order to support a regular growth of the copper material in order to fill the hole without gap formation and a subsequent failure formation. However, these chemical additives often do not reach the locations of the gaps in a sufficient amount. Rather, the chemical additives are brought into the hole (chemical source) and out of the hole again (chemical drain) without providing a large influence to the copper material growth and the gap locations.
A prior art example of such a conventional process of filling a hole in a component carrier by using chemical additives is illustrated in FIGS. 4A and 4B. FIG. 4A shows a component carrier 400 similar to the component carrier shown in FIG. 3. Indicated by arrows is a source direction 450 of chemical additives and a drain direction 451 of chemical additives. It can be seen that the chemical additives are introduced near the hole 320 in a first direction 450 and that the chemical additives drain from the hole 320 in a second direction 451. However, the flow of chemical additives does not efficiently reach the gaps 345 between the copper material portions 310. Therefore, there are still failures such as cracks and/or voids in the filled copper material 310′ in the final component carrier 400, as can be seen in FIG. 4B.
Furthermore, a prior art example of a conventional hole 520 in a conventional component carrier 500 is illustrated in FIG. 5. The thickness of the layer of electrically conductive material 310 at the bottom 333 is 4.18 μm, while the thickness of the layer of electrically conductive material 310 at the left wall 330a is 4.27 μm and the thickness of the layer of electrically conductive material 310 at the right wall 330b is 4.84 μm. Hereby, the hole 520 is 63.68 μm in height and the largest diameter of the hole 520 is 83.36 μm. Above the shoulders 332 to the left and right side of the top of the hole 520, the thickness of the layer of electrically conductive material 310 is 4.18 μm. Furthermore, it can be seen that the layer of electrically conductive material 310 on the surface of the walls 330a, 330b is not smooth at all. In summary, the thickness of the layer of electrically conductive material 310 at the walls 330a, 330b, and at the bottom 333 is very thin and irregularly shaped. As a consequence, the filling of the hole 520 with electrically conductive material is hampered and cannot be performed in an efficient manner.
Because processes using chemical additives often do not work efficiently, the performance is generally poor and failures remain. Furthermore, chemical additives lead to additional costs and are generally environmentally harmful and therefore no efficient process for failure-free filling of a hole in a component carrier is provided so far.